Uneven sealing and deformation in food packaging bags are common production challenges, primarily caused by defects in material lamination, curing processes, and improper equipment parameter control. The following provides a technical breakdown of root causes and systemic solutions:
I. Core Causes of Uneven Sealing
Lamination Process Defects
Uneven Heat Distribution: Inaccurate control of lamination temperature leads to localized overheating or insufficient cooling of the web, causing uneven film shrinkage.
Unstable Bonding: Non-uniform adhesive coating or fluctuating lamination pressure results in poor interlayer adhesion, leading to film misalignment during sealing.Curing Process Issues
Incorrect curing temperature (too high or low) disrupts adhesive cross-linking kinetics;
Insufficient curing time prevents complete drying of the adhesive, causing residual stress-induced cracking after sealing.Improper Web Stacking: (Skewed stacking) or uneven compression of webs in the curing chamber creates localized stress concentrations, inducing post-curing deformation.
Suboptimal Curing Parameters:Equipment and Mold Flaws
Defective Undercut Design: Excessive undercut depth or improper angles in molds cause uneven stress during web demolding, leading to twisting.
Inadequate Cooling System: Premature winding before the film is fully set (due to insufficient cooling time) results in post-sealing shrinkage deformation from residual heat.Material Selection Errors
Films with uneven thickness, poor heat-sealing properties (e.g., wide melting point range), or mismatched thermal shrinkage rates (between layers) fail to achieve synchronous bonding during sealing.II. Systematic Solutions
1. Optimize Lamination Process Parameters
Precise Temperature Control:
Set lamination roller temperatures according to film materials (e.g., PE: 120-150°C, PET: 130-160°C), with ±2°C tolerance. Implement gradual heating to avoid thermal shock.
Regularly calibrate temperature control systems to ensure uniformity between upper and lower rollers.
Uniform Adhesive Coating & Pressure Regulation:
Use metering rollers or comma blades to control adhesive coating weight (2-5g/m²) and prevent local accumulation or omission.
Adjust lamination pressure to 0.3-0.5MPa to ensure tight film bonding without wrinkles.2. Standardize Curing Procedures
Web Handling & Stress Relief:
Vertically hang or flat-stack webs in the curing chamber, limiting stack height to ≤1.5m with interlayer dividers for uniform stress distribution.
Allow 2-4 hours of natural cooling before curing to dissipate internal stresses from lamination.
Curing Parameter Standardization:
Set curing temperature (e.g., 50-60°C for polyurethane adhesives) and duration (24-48 hours) based on adhesive type, monitored via temperature-humidity recorders.
Conduct small-batch curing tests before mass production, verifying results through peel strength testing (≥3N/15mm).3. Equipment and Mold Modifications
Mold Structure Optimization:
Reduce undercut depth to 0.5-1.0mm and adjust demolding draft angle to 3°-5° to minimize release friction.
Install elastic silicone pads or heated sealing molds to improve sealing pressure uniformity.
Enhanced Cooling System:
Increase the number of cooling rollers or extend the cooling path to ensure film temperature drops to ≤40°C before winding.
Equip adjustable-speed cooling fans to prevent localized over-cooling and stress concentration.4. Material Selection and Pretreatment
Thermal Sealing Compatibility:
Select film combinations with ≤5% thermal shrinkage rate difference between layers (e.g., PET/PE, NY/PE). Prioritize co-extruded films or corona-treated surfaces for improved adhesion.
Preheat films at 60°C for 2 hours to reduce dimensional instability caused by hygroscopicity.
Incoming Material Inspection:
Test film thickness uniformity (≤±3% variation), heat-sealing strength (≥15N/15mm), and thermal shrinkage (longitudinal ≤5%, transverse ≤7%) upon receipt.5. Process Monitoring and Quality Control
Online Visual Inspection: Deploy vision systems to automatically capture sealing images and detect defects (e.g., wrinkles, bubbles, sealing width variation) with ≥0.1mm precision.
Sampling Verification: Perform hourly sampling (5-10 units) for sealing flatness (thickness variation ≤0.2mm via micrometer) and impact resistance testing (1.2m drop test with no cracking).III. Case Study
A food enterprise producing nut packaging bags with PET/AL/PE laminates experienced 8% sealing shrinkage (wave-like wrinkles) due to excessive curing temperature (70°C). Solutions included:
Reducing curing temperature to 55°C and extending duration to 36 hours;
Modifying mold undercut from 1.2mm to 0.8mm and increasing demolding draft angle to 4°;
Installing spray cooling on cooling rollers to maintain post-cooling film temperature at ≤35°C.
Post-adjustment, sealing flatness pass rate improved from 75% to 98%, with a 90% reduction in customer complaints.Conclusion
Addressing uneven sealing requires full-chain control across materials-process-equipment-inspection, with emphasis on standardizing critical parameters (temperature, pressure, time). For high-value products (e.g., frozen foods, vacuum packaging), implement intelligent control systems (e.g., PLC-based temperature regulation) and online defect detection to achieve zero-defect standards.
